Electroplating process

ABSTRACT

This invention provides processes for selectively electroplating metal layers into recessed topographic features on the surface of a conductive substrate. The processes are useful for fabricating metal circuit patterns, for example for creating copper interconnects between integrated circuit elements embedded in a thin layer of dielectric material on the surface of a semiconductor wafer.

FIELD OF THE INVENTION

This invention provides processes for selectively electroplating metal layers into recessed topographic features on the surface of a conductive substrate. The invention is useful for fabricating metal circuit patterns, for example for creating copper interconnects between integrated circuit elements embedded in a thin layer of dielectric material on the surface of a semiconductor wafer.

BACKGROUND

The Damascene process is currently used to fabricate copper-on-chip circuit connections known as interconnects. When this technology is applied to feature sizes below 45 nm, it can be difficult to achieve defect-free and planar metallization within the die and across the entire wafer. For example, during electroplating, areas of densely packed small features tend to overplate, creating an undesirably thick Cu layer over the features and on the plateaus that is collectively referred to as overburden.

One strategy to reduce overburden thickness and topographic variation is to manipulate the plating electrolyte composition. An electrolyte additive package typically contains accelerators (e.g., bis(3-sulfopropyl)disulfide, SPS, [Na₂(S(CH₂)₃SO₃)₂]), suppressors (e.g., chloride ions and polyethylene glycol), and levelers (e.g., Janus green B or polyethylenimine). The competitive adsorption between the accelerators and suppressors results in superfill, in which the deposition from the bottom of the trenches is accelerated relative to deposition on the openings of the trenches. However, copper bumps are formed above the copper-filled trenches.

U.S. Pat. No. 5,112,448 discloses a selective plating process whereby a photosensitive polyimide is applied to the wafer and lithographically patterned to form polymer steps between trenches. Copper is electrodeposited only into the trenches since other locations are not electrically connected to the electroplating seed layer due to the presence of the polyimide.

In another approach, the entire surface of the substrate is covered with a flexible polymeric resistive material whose flat surface contacts the plateau areas of the wafer but not the recessed circuit elements. The polymeric resistive material is chosen to be only partially permeable by the plating solution. This provides significant resistance to ionic diffusion, such that the metal ions diffuse more rapidly in the solution than through the polymeric material. Accordingly, metal ions in a volume of solution trapped in a recessed circuit feature can be plated into that feature more rapidly than metal ions can diffuse through the material to deposit onto plateau areas. While this strategy can retard the deposition of metal onto plateau areas, it can never entirely prevent it.

US2004149584 describes introducing a resistive structure between the substrate and the anode in order to reduce or eliminate the formation of plated high-points or “humps” and improve the flatness of the plated surface. JP2004225119 describes the use of a micro-porous polymer film for the same purpose.

U.S. Pat. No. 6,534,116 describes an electrochemical mechanical deposition (ECMD) process to simultaneously plate and mechanically planarize copper onto a damascene wafer in order to minimize the accumulation of copper on the plateau areas.

While the above methods can enhance the rate of metal deposition in sub-micron recesses or trenches relative to that on the plateau areas, they do not provide the desired selectivity for large recessed areas, for example, trenches or contact pads with lateral dimensions greater than 10 micron. More particularly, the above methods require that in order to completely fill large recesses with copper, excess copper must also be deposited over densely packed small features and on top of the plateau areas. Thus, there is a need for a process to selectively plate and fill large recesses without overplating densely packed small features or plateau areas.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1: Schematic illustration of membrane-limited electroplating into a topographic recess employing a cation-selective membrane with a contact-blocked electroplating barrier.

FIG. 2: Schematic drawing of the apparatus for Example 1 and Comparative Examples A and B.

FIG. 3: Schematic drawing of the apparatus for Example 2, Comparative Example C and Examples 3 and 4.

FIG. 4: Transient cell current versus time curves at fixed potential of −0.15V for electrodeposition of Cu on blank wafer test coupons using the apparatus illustrated in FIG. 2. Curves (a), (b), and (c) correspond to Example 1, Comparative Example A, and Comparative Example B, respectively.

FIG. 5: Measured weight gain per unit area after electroplating in Example 1, and Comparative Examples A and B.

FIG. 6: Transient cell current versus time at fixed potential of −0.25V for electrodeposition of Cu on blank wafer test coupons using the apparatus illustrated in FIG. 3. Curves are plotted for Example 2 and for Comparative Example C.

FIG. 7: Transient cell current vs. time for Example 3 at fixed potential of −0.25V for electrodeposition of Cu on patterned wafer test coupons using the apparatus illustrated in FIG. 3.

SUMMARY OF THE INVENTION

One embodiment of this invention is a process comprising:

-   -   a. coating a patterned conductive surface having plateau areas         and recessed features with a precursor comprising an N—, S— or         O-containing polymer, oligomer or dendrimer selected from the         group consisting of polyethylenimine, polypropylenimine,         polyaniline,         N,N,N′,N′-tetrakis(3-aminopropyl)-1,4-butanediamine,         poly(3,4-ethylenedioxythiophene), polyethyleneoxide, and         polycation electrolytes, forming a precursor-coated conductive         surface;     -   b. providing a metal-exchanged cation-conducting membrane         comprising a first surface and an opposing second surface,         wherein the cation-conducting membrane is permeable to platable         metal cations including silver, nickel, cobalt, tin, aluminum,         copper, lead, tantalum, titanium, iron, chromium, vanadium,         manganese, zinc, zirconium, niobium, molybdenum, ruthenium,         rhodium, hafnium, tungsten, rhenium, osmium, iridium, palladium,         platinum, gold, and indium cations and mixtures thereof;     -   c. contacting the precursor-coated conductive surface with the         first surface of the metal-exchanged cation-conducting membrane         to provide a electroplating barrier at the plateau areas;     -   d. providing an anode in electrical contact with the anolyte;     -   e. providing an anolyte composition comprising a solution of one         or more metal cations selected from the group consisting of         silver, nickel, cobalt, tin, copper, lead, iron, chromium,         manganese, zinc, ruthenium, rhodium, rhenium, osmium, palladium,         platinum, hafnium, gold, indium, and iridium cations and         mixtures thereof, wherein the anolyte composition is in contact         with the anode and the second surface of the membrane; and     -   f. applying a voltage between the anode and the patterned         conductive surface to electroplate at least a portion of the         metal cations onto the precursor-coated surface to form a metal         layer in the recessed features.

DETAILED DESCRIPTION

One embodiment of this invention is a process for selectively electroplating metal into the recessed areas of a macroscopically flat substrate containing 0.01 micron to 1,000 micron recessed topographic features, while substantially minimizing or entirely avoiding the deposition of metal onto the intervening plateau areas. This result is achieved by selectively forming an electroplating barrier on the plateaus by contacting a coating of a precursor on the wafer with a metal ion-loaded ion exchange membrane.

The selectivity of the electroplating barrier can be varied, for example, by controlling the electrodeposition potential during the electroplating process.

The processes of this invention can be used to selectively electroplate a wide variety of platable metals and metal alloys into recessed areas of a macroscopically flat subsurface. Suitable metals include silver, nickel, cobalt, tin, aluminum, copper, lead, tantalum, titanium, iron, chromium, vanadium, manganese, zinc, zirconium, niobium, molybdenum, ruthenium, rhodium, hafnium, tungsten, rhenium, osmium, iridium, palladium, platinum, gold, indium, and combinations thereof. Preferred metals include silver, nickel, cobalt, tin, aluminum, copper, lead and combinations thereof. The processes are particularly suitable for electroplating copper and/or copper-containing alloys onto damascene wafers.

Definitions

Unless otherwise stated, the following terms when used herein have the meanings set forth below.

In the present specification, “trenches,” “recessed features,” “holes,” “recessed trenches,” “topographic recesses,” and “vias,” may be used alternatively, in conjunction, selectively, or interchangeably. Unless otherwise specified, usage of any of these terms includes every type of recessed feature that is not a plateau and the meaning is construed to comprehensively include all types of features.

By “plateau,” is meant a generally flat area of the substrate or conductive surface that is at the level of the top of the trenches and/or vias.

Unless otherwise specified, a “conductive” fluid or solution has conductivity greater than about 5 mS/cm, preferably at least about 30 mS/cm, more preferably at least about 100 mS/cm.

A “conductive surface” has a sheet resistance no greater than about 110 milli-ohms per square.

An “oligomer” is a low-molecular weight polymer, typically with two to five monomer units.

A “dendrimer” is a macromolecule made of branched repeat units arranged in layers emanating from a point-like core.

One embodiment of a process of this invention is conveniently carried out using an electroplating cell as schematically shown in FIG. 1. The surface of the substrate 10 (the conductive surface) is initially coated with the precursor. A first surface of the metal ion-conductive membrane 13 is then brought into contact with the coated precursor in the plateau regions, and the reaction between the precursor and the membrane forms a barrier layer 18 on the plateau regions. The membrane also displaces the plating solution from the plateau surfaces while leaving small volumes of electroplating solution trapped in the recessed features 12. The membrane can be held or pressed against the surface of the substrate by hydrostatic pressure of the anolyte solution 16. When a suitable potential difference is applied between the anode 15 and the substrate surface 11, dissolved metal ions 9 (for example, Cu²⁺) within the recessed features are reduced to metal 17 (for example, Cu⁰) which plates onto the recessed features.

The metal ions that are reduced and plate out are replaced by metal ions that migrate from the anolyte through the metal ion-conductive membrane, thereby preventing the electroplating solution in the trenches or from being completely depleted of platable metal ions. In contrast, substantially no metal ions migrate from electroplating solution disposed in recessed features to the plateau surfaces, and substantially no metal ions from the anolyte can reach the plateau surfaces through the membrane due to the plating barrier layer 18 formed there. As a result, little or no metal plates onto the plateau surfaces. The net result of these processes is that metal plates selectively onto the recessed features.

An embodiment of an electroplating cell comprises a substrate to be lo electroplated, electroplating solution in contact with the substrate, and an anodic half-cell with a metal ion-conducting membrane attached that can be brought into contact with the substrate. The substrate also serves as the cathode. The anodic half-cell comprises a metal anode in contact with an anolyte and a metal ion-conducting membrane that has a first surface that can be brought into contact with the substrate and/or electroplating solution and a second surface in contact with the anolyte. A power source is connected to the metal anode and the substrate cathode.

Cell Configuration as Illustrated in the Figures

FIG. 2 illustrates one embodiment of a process of the present invention wherein a copper foil anode 24, a Cu²⁺-conducting Nafion®117 membrane 21 (E.I. du Pont de Nemours, Inc., Wilmington, Del.), the test coupon cathode 20, a 50 mm×50 mm silicone rubber square 22 with a 16 mm diameter through-hole in the center 23, and two 50 mm×50 mm polycarbonate plates with copper foil tabs 25 are pressed together by a mechanical clamp. The Nafion® membrane can be pretreated in 1.0 M CuSO₄/1.5M H₂SO₄ solution for 24 hrs to convert it into its Cu²⁺ form. Cu²⁺-containing electrolyte flows through the center hole by passing through the flow channel 26. A Solartron 1287 potentiostat (Solartron Inc, Houston, Tex.) is electronically connected to the assembly.

FIG. 3 illustrates another embodiment of a process of the present invention. The electroplating cell contains an anode membrane cell 27 with a copper disk anode 28, an electrolyte compartment 29, a ceramic honeycomb 30 (Versagrid, Applied Ceramic Inc, Doraville, Ga.), and a Cu²⁺-conducting Nafion®117 membrane 31. The membrane cell has ports 32 linked to an electrolyte reservoir through a pump and plastic tubing so that the electrolyte can be supplied and circulated to the electrolyte compartment. A sample fixture 33 holds the test coupon 34, which serves as the cathode. The Nafion® membrane can be soaked in a solution containing 0.5 M CuSO₄ and 1.5 M H₂SO₄ solution for 24 hr prior to be mounted in the membrane cell.

DC Power Source

To drive the electroplating process, a source of DC electrical power is connected between the substrate (which functions as the cathode) and the metal anode. The source of DC power can be steady or can advantageously provide pulses and/or variable DC power. A typical Damascene wafer comprises a seed layer of metal, for example copper, that covers the Ta or TaN barrier layer to provide the electrical connection to the power source

Anode

The anode is an electrically conductive material such as a metal or alloy (e.g., copper, nickel, silver, palladium, platinum or dimensionally-stable anode commonly used in the chlor-alkali process) or carbon. Typically, the anode comprises the same metal that is being electroplated. For example, the anode is made of copper if the electroplating process is to plate copper onto the precursor-coated surface. Electrical contact between the anode and the second surface of the membrane can beneficially be through an anolyte contacting the anode and the second surface of the membrane.

Anolyte

The anolyte acts as a source and/or sink for ions passing through the membrane. Suitable anolyte compositions can be selected from water, polar organic solvents, and/or combinations thereof. Suitable organic solvents include methanol, propanol, butanol, tetrahydrofuran, 1,3-Dioxolane, acetonitrile, dimethylsuloxide, dimethylacetamide, propylene carbonate, and Fluorinert™ liquids and their mixtures. The anolyte can also contain solutes such as acids, bases or salts. Higher conductivity anolyte compositions, for example compositions having a conductivity of at least 20 mS/cm, are generally preferred, as they can reduce voltage loss of current passing from the anode through the anolyte. The anolyte also typically contains cations of the metal to be plated onto the precursor-coated surface, e.g., cations derived from copper sulfate salts.

Metal Ion-Conducting Membrane

Suitable ion-conducting membranes are permeable to the platable metal ions in the anolyte solution. The ion-conducting membrane serves three functions. The first function is to displace solution from the plateau areas of the conductive surface while trapping solution within the recessed areas. The second function of the membrane is to react with the precursor to form a polymer complex that is non-conductive to the platable metal ions. The third function of the membrane is to conduct metal ions from the anolyte to the conductive surface to be plated.

Suitable ion-conducting membranes include film-forming ionic polymers that are stable under the conditions of the electroplating process. Ionic polymer membranes useful in electro-coating, electro-dialysis, the chloralkali process and/or fuel-cells can be used in the electroplating processes described herein. Typically, the thickness of the ion-conducting membrane is greater than the width of trenches to be filled with electroplated metal. In some embodiments, the membrane thickness is at least 2 times the largest width of trenches to be filled with electroplated metal. The membrane is typically from about 10 microns to about 500 microns thick. The membrane should be thick and/or stiff enough to resist bending so that it does not conform to the topography of the conductive surface. The distribution of charged moieties in the pores need not be uniform. The membrane can comprise one or more separate membranes laminated one to another.

Reinforced exchange polymer membranes can also be used in the practice of the present invention. Reinforced membranes can be made by impregnating porous, expanded PTFE (ePTFE) with ion exchange polymer. Tetratex® ePTFE is available from Tetratec, Feasterville Pa., and Goretex® ePTFE is available from W. L. Gore and Associates, Inc., Elkton Md. Impregnation of ePTFE with perfluorinated sulfonic acid ionomer is disclosed in U.S. Pat. No. 5,547,551 and U.S. Pat. No. 6,110,333. The reinforcement provides increased strength and permits use of thinner membranes, and also contributes to greater dimensional stability of the membrane.

The stiffness and compressibility of the membrane can vary with degree of saturation as well as the ion content in the membrane, but generally, most commercially available Nafion® and Flemion® membranes have the requisite stiffness and incompressibility when the saturation level is near 100%.

Cation-Conductive Membranes

In the processes of this invention, a cation-conducting membrane is used. An exemplary cation-conducting membrane comprises a polymeric ionomer functionalized with at least one type of acidic moiety. Cation-conducting membranes (also called cation-exchange membranes), generally comprise organic polymer films with acidic functional groups (e.g., —CO₂H, —SO₃H or phosphonate groups), bound covalently to the polymeric backbone, wherein the functional groups are capable of complexing with the ionic form of the metal to be plated. In one embodiment, the cation-conducting membranes are formed from polymeric ionomers functionalized with strong acid groups that have a pKa of less than about 3. Sulfonic acid groups are preferred strong acid groups. Preferred polymeric ionomers are copolymers of fluorinated and/or perfluorinated olefins and monomers containing strong acid groups. “Fluorinated” means that at least 10% of the univalent atoms in the polymer are fluorine atoms. “Highly fluorinated” means that at least 90% of the univalent atoms in the polymer are fluorine atoms. “Perfluorinated” means that essentially 100% of the univalent atoms are fluorine atoms.

An exemplary cation-conducting membrane can have 1.0 to 4.0 milli-equivalents of strong acid groups per cubic centimeter of membrane. Suitable membranes include polytetrafluoroethylene-based membranes, perfluorocarboxylic acid/polytetrafluoroethylene copolymers, polymeric ionomers functionalized with both sulfonic acid groups and carboxylic acid groups, and perfluorosulfonic acid/polytetrafluoroethylene copolymer membranes. Other acid moieties can be attached to the membrane as an lo alternative to, or in addition to, the carboxylic acid moieties and/or the sulfonic acid moieties, including for example sulfanilamide moieties, phosphonate moieties, sulfonyl moieties, or any combination thereof, wherein the acidic moieties can independently be substituted with, for example, a C₁ to C₄ alkyl group.

In one embodiment of this invention, the polymer used in the cation-conducting membrane comprises a polymer backbone with recurring side chains attached to the backbone, with the side chains carrying the acid groups. Suitable polymers include homopolymers, and copolymers of two or more monomers. Copolymers are typically formed from at least one first monomer which contains no functional side-groups and a second monomer containing side chains carrying the acid group or its precursor, e.g., a sulfonyl halide group such as sulfonyl fluoride (—SO₂F), which can be subsequently hydrolyzed and converted to a sulfonate group. Suitable first monomers include tetrafluoroethylene (TFE), hexafluoropropylene, vinyl fluoride, vinylidine fluoride, trifluoroethylene, chlorotrifluoroethylene, perfluoro(alkyl vinyl ether), hexafluoroisobutylene ((CH₂═C(CF₃)₂), ethylene, and mixtures thereof. Suitable second monomers include fluorinated vinyl ethers with sulfonate groups or precursor groups that can provide the desired side chain in the polymer. Copolymers of a first fluorinated vinyl monomer together with a second fluorinated vinyl monomer having a sulfonyl fluoride group (—SO₂F) are suitable. Other monomers can also be incorporated into these polymers if desired.

Other sulfonate ionomers can also be used to make the membrane polymers, for example trifluorostyrene bearing sulfonate groups on the aromatic rings (U.S. Pat. No. 5,773,480). Alternatively, the trifluorostyrene monomer can be grafted to a base polymer to make the ion-exchange polymer (U.S. Pat. No. 6,359,019).

In one embodiment of this invention, the membrane polymer contains a highly fluorinated or perfluorinated carbon backbone and a side chain represented by the formula —(O—CF₂CFR_(f))_(a)—O—CF₂CFR′_(f)SO₃CF₃, wherein a=0,1 or 2, and R_(f) and R′_(f) are independently selected from F, Cl 10 and a perfluorinated alkyl group having 1 to 10 carbon atoms. Suitable polymers are disclosed in U.S. Pat. No. 3,282,875, U.S. Pat. No. 4,358,545 and U.S. Pat. No. 4,940,525. One suitable polymer comprises a perfluorocarbon backbone with side chains represented by the formula —O—CF₂CF(CF₃)—O—CF₂CF₂SO₃CF₃. Polymers of this type are disclosed in U.S. Pat. No. 3,282,875 and can be made by copolymerization of TFE, CF₂═CF—O—CF₂CF(CF₃)—O—CF₂CF₂SO₂F (1) (a perfluorinated vinyl ether), and perfluoro(3,6-dioxa-4-methyl-7-octenesulfonyl fluoride) (PDMOF), followed by conversion of the sulfonyl halide groups to sulfonate groups by hydrolysis and optional ion exchange. Suitable polymers with side chains of —O—CF₂CF₂SO₃CF₃ are disclosed in U.S. Pat. No. 4,358,545 and U.S. Pat. No. 4,940,525. This polymer can be made by copolymerization of TFE and the perfluorinated vinyl ether CF₂═CF—O—CF₂CF₂SO₂F (2), and perfluoro(3-oxa-4-pentenesulfonyl fluoride), followed by hydrolysis and ion exchange.

Polymers used in the cation-conducting membranes are typically characterized by their equivalent weight, which is the weight of polymer in the hydrogen-ion or acid form in grams that will neutralize one equivalent of base. For the sulfonate polymers described above, equivalent weights are in the range of 700 to 1500, preferably about 800-1350, more preferably about 850 to 1200, most preferably about 900 to 1100. Alternatively, the polymers can be characterized by their ion-exchange ratio (IXR), which is the number of carbon atoms in the polymer backbone divided by the number of ion-exchange groups. The IXRs corresponding to the EW ranges given above are about 8 to 24, preferably about 10 to 21, more preferably about 12 to 18. For the copolymers of TFE and vinyl ether (1), IXR is related to equivalent weight (EW) by the equation: (EW=50×IXR+344). For the copolymers of TFE and vinyl ether (2), IXR is related to equivalent weight (EW) by the equation: (EW=50×IXR+308).

Commercially available cation-conducting membranes useful in the processes of this invention include Flemion® perfluorocarboxylate ionomer membranes (Asahi Glass Co., Ltd, Yokahama, Japan) and/or Nafion® perfluorosulfonate membranes (E.I. du Pont de Nemours, Inc., Wilmington, Del.), which are composed of fluorocarbon chains bearing highly acidic carboxylic and sulfonic acid groups, respectively. On exposure to water, the acid groups of the perfluorosulfonate ionomer ionize, leaving fixed sulfonate anions and mobile hydrated protons. The protons may be readily exchanged with various metal cations. Nafion® perfluorosulfonate membranes are particularly well-suited for use in membrane-limited selective electroplating due to theirstrong common-ion exclusion, high conductivity, strong acidity, chemical stability and robust mechanical properties.

Cation-conducting membranes employed in the electroplating cell in accordance with the invention are about 5 μm to about 250 μm thick, or about 10 to about 200 μm thick, or about 100 to about 150 μm thick.

In one embodiment, the membrane is layered and comprises a fluoropolymer membrane comprising at least two integrally laminated layers including a first layer made of a perfluorocarbon polymer having carboxylic acid groups, and a second layer comprising perfluorocarbon polymer having sulfonic acid groups.

Alternatively, the layers can be separated by a fluid layer. A suitable membrane can be a single layer having both sulfonic and carboxylic groups made, for example, by the copolymerization of a carboxylic acid type monomer with a sulfonic acid type monomer, or by the copolymerization of a carboxylic acid type monomer with a sulfonic acid type monomer, or by impregnating a sulfonic acid type fluoropolymer membrane with a carboxylic acid type monomer, followed by polymerization. Suitable membranes include those formed from a blend comprising a sulfonic acid group-containing polymer and a carboxylic acid group-containing polymer, which is laminated on a sulfonic acid group membrane, as described in U.S. Pat. No. 4,176,215, and herein incorporated by reference.

Prior to use in the processes of this invention, the cation-conductive membrane is converted to the metal-exchanged form, e.g., by lo soaking the membrane in an appropriate solution containing a metal salt. For Cu⁺²-conductive membranes, an acidic solution of copper sulfate can be used.

Electroplating Solution

The processes of this invention can employ conventional electroplating solutions comprising salts of metal ions or complex metal ions and other ingredients, for example plating additives, acids or bases, buffers, and surfactants. Any or all additives known for use in electroplating solutions can be used in the processes including various additives designated as “accelerators”, “suppressors” “levelers”, and “brighteners”. Various additives effective for metallization of wafers can be used in the present invention. Examples include additive packages that enable bottom-up fill of recessed features through surface interactions between the additives.

Any apparatus or device suitable for supplying an electroplating solution containing additives to an area between a membrane and a substrate is useful, including, for example, baths and sprayers. The electroplating solution can be supplied at any pressure, and the supply can be intermittent or continuous. The electroplating solution can be of one composition, or the composition can be changed during the plating process.

Substrate/Cathode

Substrates useful in the processes of this invention include wafers used in the semiconductor industry to manufacture integrated circuits. Typically, the wafers have flat plateau regions as well as recessed features such as trenches and vias. The wafers are typically made of silicon, and are coated with a layer of a barrier material such as Ta or TaN, and then overcoated with a seed layer of another metal to provide a conductive surface. The conductive surface typically comprises a metal such as copper, but other metals may be used as long as they provide suitable electrical contact to the power source and adhesion to the plated copper.

Preparation of Coated Substrate

In the processes of this invention, the wafer surface is coated with a solution of a polymer, herein referred to as the precursor, and then the wafer is dried to remove the solvent.

Processes to coat the wafer with the precursor include dip-coating and spin-coating. A spin-coating solution typically consists of 1 to 10 wt % polymer, and more typically 2 to 5 wt %. The solvent can be water, an organic solvent, or a mixture of solvents in which the polymer is soluble. In one embodiment, the solvent is an aqueous mixed solvent. In this embodiment, the water to organic solvent ratio is chosen so that the advanced contact angle of the polymer solution with the conducting substrate is zero. Suitable organic solvents include alcohols such as methanol, ethanol, butanol, acetone, N-methylpyrolidone, dimethylformamide, dimethylsulfonamide, and dimethyl carbonate.

The thickness of the film is typically 1 to 1000 nm, or 10 to 100 nm. The thickness is determined by the precursor concentration (higher concentrations lead to thicker coatings), pH, and the number of coating layers (more coatings provide thicker films). The film can be a single layer or a multi-layer film of 2 to 50 layers.

If it is single layer, the single layer is an N—, S— or O-containing polymer, oligomer or dendrimer selected from the group consisting of polyethylenimine, polypropylenimine, polyaniline, N,N,N′,N′-tetrakis(3-aminopropyl)-1,4-butanediamine, poly(3,4-ethylenedioxythiophene), polyethyleneoxide, and polycation electrolytes. Suitable polycation electrolytes include polyallylamine hydrochloride, poly(4-vinyl-1-pyridinium bromide) and poly(2-vinyl-1-methylpyridinium bromide).

If more than one layer, a polyanion electrolyte layer and an N—, S— or O-containing polymer, oligomer or dendrimer alternate within the layers, but the outermost layer facing the cation exchange membrane is an N—, S— or O-containing polymer, oligomer or dendrimer selected from the list of polyethylenimine, polypropylenimine, polyaniline, N,N,N′,N′-tetrakis(3-aminopropyl)-1,4-butanediamine, poly(3,4-ethylenedioxythiophene), polyethyleneoxide, and polycation electrolytes.

Suitable polyanion electrolytes include sulfonated polystyrene (PSS), polyacrylic acid (PAA), and polymethylacrylic acid (PMAA).

Suitable electrostatically-built multi-layer films that may be useful as the precursor for the prevent invention are described by Bertrand et al, Macromol. Rapid Communications, 21, 319 and references cited therein.

Electroplating Process

After the substrate is coated with the precursor and the solvent is removed, the metal-loaded membrane is brought into contact with the coated precursor, and the precursor reacts with the membrane to form a polymer complex with the membrane that is nonconductive to the metal. This step is conducted in the presence of electroplating solution. It is not necessary to apply a voltage for the polymer complex to form. The reaction of the precursor with the membrane only occurs on the plateau area of the wafer surface where there is direct contact between the coated precursor and the membrane. The polymer complex that is formed is referred to as a “barrier layer,” indicative of its role in preventing metal from plating onto the plateau areas. Because the membrane does not contact the precursor-coated regions of the recessed areas, no barrier layer is formed in the recessed areas. As a result, metal transport to the plateau is blocked during electroplating, while that to the recessed features is not.

When the membrane is brought into contact with the substrate, there may be a thin layer of fluid disposed between the membrane and the wetted plateaus; preferably there is none. Some solution remains trapped within the recessed features. The trapped electroplating solution within the recesses serves as an initial source of metal ions in an electroplating step. Additional metal ions are supplied by the continuous migration of metal ions from the anolyte through the ion-conducting membrane.

The electroplating step is carried out while the membrane is still in contact with the substrate. When the membrane is held against the substrate and a suitable voltage is applied between the anode and cathode, metal ions of the electroplating solution are reduced to elemental metal and are deposited into the trenches of the conductive surface. Unlike conventional electroplating, a disproportionately large portion of the electrical current flows through small volumes of the electroplating solution trapped within the topographic recesses (trenches) of the surface. The entrapment of the electroplating solution is achieved by holding the conductive surface in contact with the membrane. During the electroplating process, there can be a substantial change in concentrations of constituents in the anolyte solution. In order to maintain uniform processing conditions, it may be advantageous to remove and replace used solutions from the anode compartment, or to otherwise affect the composition to maintain stable concentrations in the anolyte solution.

Electroplating can be carried out under constant voltage, constant current or pulsed modes. For example, the electroplating can be a constant voltage at cell potential of 0.1 to 0.3 V, or 0.15 to 0.25 V, in which the wafer side is the negative terminal. During electroplating, the wafer and the electrochemical half-cell do not move relative to each other, and the membrane stays in direct contact with the wafer surface. Because the barrier layer forms only on the plateau areas, metal ion transport and electroplating occur only in the areas of the recessed features, and there is little or no overburden.

After electroplating, the membrane can be immersed in electrolyte solution (e.g., acidic Cu⁺² solution) to restore high levels of metal-loading in the membrane.

Although the present invention is described with reference to certain preferred embodiments, it is apparent that modification and variations thereof may be made by those skilled in the art without departing from the spirit and scope of this invention as defined by the lo appended claims. In particular, it will be clear to those skilled in the art that the present invention may be embodied in other specific forms, structures, arrangements, proportions, and with other elements, materials, and components, without departing from the spirit or essential characteristics thereof. One skilled in the art will appreciate that the invention may be used with many modifications of materials, methods, and components otherwise used in the practice of the invention, which are particularly adapted to specific substrates and operative requirements without departing from the principles of the present invention. The presently disclosed embodiments are therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims, and not limited to the foregoing description.

EXAMPLES Example 1

Electroplating Cu onto Blank Wafer Coated with a Plating Barrier Precursor

An approximately 2.5 cm×2.5 cm sized test coupon was cut from a 200 mm blank test wafer (Advanced Technology Development Facility Inc, Austin, Tex.). The as-received wafer had a Ta barrier layer, a Cu seed layer, and an electroplated Cu layer. The test coupon was pre-treated first by acid washing (0.5 M H₂SO₄/0.5 M HNO₃), followed by washing with de-ionized water then dried in a vacuum oven at 110° C. for 30 min. Before use, the coupon was immersed in acetic acid (99.9%) for 1 min and dried under a N₂ stream for 5 min.

The test coupon was placed on the chuck of a spin coater (Spin coat G3P, Speedline Technologies, Indianapolis, Ind.). A plating precursor consisting of about 0.5 mL solution of 4 wt % poly(1-methyl-2-vinylpyridinium bromide) (Polysciences Inc., Warrington, Pa.) in a methanol:water mixture (4:1 by volume) was dripped with a pipette on the center of the test coupon. Then the sample was rotated at 2500 rpm for approximately 60 sec. The sample was taken out of the spin coater and dried under a N₂ stream for 5 min.

FIG. 2 shows the electroplating cell, which consists of a copper foil anode, a pretreated Nafion®117 membrane (E.I. du Pont de Nemours, Inc., Wilmington, Del.), the test coupon as the cathode, a 50 mm×50 mm silicone rubber square with a 16 mm diameter through-hole in the center, and two 50 mm×50 mm polycarbonate plates. The Nafion® membrane had been pretreated in 1.0 M CuSO₄/1.5M H₂SO₄ solution for 24 hr to convert it into its Cu²⁺ form. A mechanical clamp pressed the parts together to form the assembly depicted in FIG. 2. The electroplating apparatus further consists of a Solartron 1287 potentiostat (Solartron Inc, Houston, Tex.) and a computer. The potentiostat was connected to the electroplating cell such that its CE (counter electrode) was connected to the test coupon (cathode) and the WE (working electrode) was connected to the copper foil anode. Electrolyte (0.25 M CuSO₄/1.0M H₂SO₄) was passed through the cell at a rate of about 1 mL/min. CorrWare® software (Scribner Associates Inc., Southern Pines, N.C.) was used to program and control the potentiostat. The plating was potentiostatically controlled by imposing 0.15 V for 500 sec. The current vs. time curve for the sample is shown in FIG. 4 (curve a). After the plating, the test coupon was dried under a N₂ stream for 5 min. The weight of the coupon was unchanged (FIG. 5). This demonstrates that with a plating barrier layer, there was no detectable copper deposition.

Comparative Example A

Electroplating Cu onto a Blank Wafer without a Plating Barrier Precursor

The test coupon, pretreatment conditions, and electroplating apparatus are the same as Example 1 except that the test coupon was not spin-coated with poly(1-methyl-2-vinylpyridinium bromide).

The plating was potentiostatically controlled by imposing 0.15 V for 500 sec. The current vs. time curve for the sample is shown in FIG. 4 b. After the plating, the test coupon was dried under a N₂ stream for 5 min. The weight gain was 0.401 mg/cm² as shown in FIG. 5. The charge passed in the electroplating process was 8.26 col, which is equivalent to 0.403 mg/cm² Cu. The Cu electroplating efficiency is about 99.3%. This demonstrates that Cu deposition readily occurs through a Nafion® membrane in the absence of a plating barrier.

Comparative Example B

Electroplating Cu onto a Blank Wafer using a Liquid Plating Electrolyte in the Presence of a Plating Barrier Precursor

The test coupon and pretreatment conditions are the same as for Example 1 and Comparative Example A.

The electroplating apparatus was the same as Example 1 except there was no Nafion® membrane so that the liquid electrolyte was in direct contact with the test coupon during electroplating. The liquid electrolyte cell passed through the cell at a rate of 1 cc/min, in direct contact with the test coupon. The liquid electrolyte contained 0.25 M CuSO₄, 1.0 M H₂SO₄, and 0.1 g/L bis(sodium sulfopropyl) disulfide, SPS, (Na₂(SO₃(CH₂)₃S)₂) (Raschig Chemicals GmbH, Ludwigshafen, Germany). The electroplating was potentiostatically controlled by imposing 0.15 V for 500 sec. The current vs. time curve for the sample is shown in FIG. 4 (curve c). After the plating, the test coupon was washed with water and dried under a N₂ stream for 5 min. The weight gain was 0.38 mg/cm² (FIG. 5). The charge passed in the electroplating process was 1.76 col, which is equivalent to 0.44 mg/cm² of electrodeposited Cu. The Cu electroplating efficiency was about 86%.

FIG. 5 compares the Cu deposition weight of Example 1, and Comparative Examples A and B. These examples demonstrate that the Nafion® membrane in contact with the barrier precursor polymer forms a plating barrier layer, and that the precursor polymer alone does not function as a plating barrier.

Example 2

Electroplating Cu onto a Blank Cu Disk Coated with a Plating Barrier lo Precursor

A 0.812 mm thick 37.5 mm×37.5 mm blank pure Cu disk was acid-washed (0.5 M H₂SO₄/0.5 M HNO₃), rinsed with de-ionized water, and then dried in a vacuum oven at 110° C. for 30 min. Before use, the coupon was immersed in acetic acid (99.9%) for 1 min and then dried under a N₂ stream.

The test coupon was spin-coated with 1 mL solution of 4 wt % poly(1-methyl-2-vinylpyridinium bromide) and dried, as described in Example 1.

A schematic of the electroplating cell is shown in FIG. 3. For this example, the electroplating cell contained an anode membrane cell with a copper disk anode, an electrolyte compartment, a ceramic honeycomb (Versagrid, Applied Ceramic Inc, Doraville, Ga.), and a Nafion®117 membrane. The membrane cell had ports linked to an electrolyte reservoir through a pump and plastic tubing so that the electrolyte could be supplied and circulated to the electrolyte compartment. A sample fixture held the test coupon, which served as the cathode. The Nafion® membrane was soaked in a solution containing 0.5 M CuSO₄ and 1.5 M H₂SO₄ solution for 24 hr. A Solatron 1287 potentiostat was connected to the cell such that the CE (counter electrode) was connected to the test coupon (cathode) and the WE (working electrode) was connected to the copper disk anode. CorrWare® software (Scribner Associates Inc., Southern Pines, N.C.) was used to program and control the potentiostat.

The spin-coated copper disk was placed in the cathode fixture and was covered with a solution containing 0.25M CuSO₄ and 250 μM SPS in water. Then, the membrane cell was brought into contact with the copper sample surface. The plating was potentiostatically controlled by imposing 0.25 V for 180 sec. The current vs. time curve for the sample is shown in FIG. 6. After the plating, the test coupon was dried under a N₂ stream for 5 min. The weight gain of the coupon was 0.4 mg. This demonstrates that with a plating barrier layer, there was little copper deposition.

Comparative Example C

Electroplating Cu onto Blank Cu disk without a Plating Barrier Precursor

The sample, pretreatment conditions, and electroplating apparatus are the same as Example 2 except that the test coupon was not coated with the poly(1-methyl-2-vinylpyridinium bromide) polymer solution.

After the copper disk is placed in the cathode fixture, it was covered with solution containing 0.25M CuSO₄ and 250 μM SPS in water. Then, the membrane cell was brought into contact with the copper sample surface. The plating was potentiostatically controlled by imposing 0.25 V for 180 sec. The current vs. time curve for the sample is shown in FIG. 6. After the plating, the test coupon was dried under a N₂ stream for 5 min. The weight gain of the test coupon was 14.5 mg, which is much more than that in Example 2. This demonstrates that without the plating precursor Cu deposition through a Nafion® membrane is much more rapid than when the sample is coated with a plating precursor.

Example 3

Electroplating Cu onto a Patterned Damascene Silicon Wafer Coated with a Plating Barrier Precursor

A 200 mm Damascene patterned test wafer with a Ta barrier layer, a Cu seed layer, and a 1.5 μm electroplated Cu layer (Product ID 854, Advanced Technology Development Facility Inc, Austin, Tex.) was cut into approximately 40.3 mm×40.3 mm square test coupons. A test coupon was acid washed (0.5 M H₂SO₄/0.5 M HNO₃), rinsed with de-ionized water, and then dried in a vacuum oven at 110° C. for 30 min. Before use, the coupon was immersed in acetic acid (99.9%) for 1 min and dried under a N₂ stream.

The test coupon was spin-coated with 1 mL of 4 wt % poly(1-methyl-2-vinylpyridinium bromide) and dried, as described in Example 1.

The test coupon was used as the cathode in an electroplating cell, as described in Example 2.

After the patterned test coupon was placed in the cathode fixture, it was covered with a solution containing 0.25M CuSO₄ and 250 ρM SPS in lo water. Then, the membrane cell was brought into contact with the copper sample surface. The plating was potentiostatically controlled by imposing 0.25 V for 180 sec. The current vs. time curve for the sample is shown in FIG. 7. After the plating, the test coupon was dried under a N₂ stream for 5 min. The weight gain of the coupon was 2.1 mg, indicating Cu electroplating took place. 

1. A process comprising: a. coating a patterned conductive surface having plateau areas and recessed features with a precursor comprising an N—, S— or O-containing polymer, oligomer or dendrimer selected from the group consisting of polyethylenimine, polypropylenimine, polyaniline, N,N,N′,N′-tetrakis(3-aminopropyl)-1,4-butanediamine, poly(3,4-ethylenedioxythiophene), polyethyleneoxide, and polycation electrolytes, forming a precursor-coated conductive surface; b. providing a metal-exchanged cation-conducting membrane comprising a first surface and an opposing second surface, wherein the cation-conducting membrane is permeable to platable metal cations including silver, nickel, cobalt, tin, aluminum, copper, lead, tantalum, titanium, iron, chromium, vanadium, manganese, zinc, zirconium, niobium, molybdenum, ruthenium, rhodium, hafnium, tungsten, rhenium, osmium, iridium, palladium, platinum, gold, and indium cations and mixtures thereof; c. contacting the precursor-coated conductive surface with the first surface of the metal-exchanged cation-conducting membrane to provide a electroplating barrier at the plateau areas; d. providing an anode in electrical contact with the anolyte; e. providing an anolyte composition comprising a solution of one or more metal cations selected from the group consisting of silver, nickel, cobalt, tin, copper, lead, iron, chromium, manganese, zinc, ruthenium, rhodium, rhenium, osmium, palladium, platinum, hafnium, gold, indium, and iridium cations and mixtures thereof, wherein the anolyte composition is in contact with the anode and the second surface of the membrane; and f. applying a voltage between the anode and the patterned conductive surface to electroplate at least a portion of the metal cations onto the precursor-coated surface to form a metal layer in the recessed features.
 2. The process of claim 1, wherein the precursor is a polycation electrolyte selected from the group consisting of polyethyleneimine, polyallylamine hydrochloride, poly(4-vinyl-1-pyridinium bromide) and poly(2-vinyl-1-methyl pyridinium bromide).
 3. The process of claim 1, wherein the precursor is an N—, S— or O-containing polymer, oligomer or dendrimer selected from the group consisting of polyethylenimine, polypropylenimine, polyaniline, N,N,N′,N′-tetrakis(3-aminopropyl)-1,4-butanediamine, poly(3,4-ethylenedioxythiophene), and polyethyleneoxide.
 4. The process of claim 1, wherein the metal-exchanged cation-conducting membrane is selected from the group consisting of metal-exchanged perfluorocarboxylate ionomer membranes and metal-exchanged perfluorosulfonate ionomer membranes.
 5. The process of claim 4, wherein the metal-exchanged cation-conducting membrane is formed by contacting a cation-conducting membrane with an acidic solution of a copper salt.
 6. The process of claim 1, wherein the patterned conductive surface is the surface of a Damascene wafer.
 7. The process of claim 1, wherein the anolyte composition comprises a solution of copper cations.
 8. The process of claim 1, wherein the metal layer comprises copper. 